In recent years, DVD and CD (compact disc) are widely used as an optical disk type-recording medium, and an optical disk drive which is capable of reproducing both the DVD and CD is becoming pervasive. Since there is a difference in recording density, disk substrate thickness, or the like, between the DVD and CD, it is necessary to irradiate such optical disks respectively with laser beams having different properties, so as to reproduce data on those disks appropriately.
In view of the point above, an optical disk drive being capable of reproducing both DVD and CD has a function to find out whether the optical disk being loaded is CD or DVD.
Japanese Patent Laid-open Publication No. Hei 5-54406 discloses a technique as an optical disk discrimination method in which a length of time taken for a focus to move from a disk surface to a recording layer is measured from a focus error signal obtained while an objective lens approaches the optical disk at a constant speed. Based on thus measured length of time, the type of the optical disk is determined. This technique utilizes a feature that there is a difference in disk substrate thickness between CD and DVD, and thus a distance from the disk surface to the recording layer is also different.
In order to prevent an error due to a change in relative position between the objective lens and the optical disk, caused by in-periphery surface wobbling at the time of the optical disk rotation, the optical disk should be in halt condition when the type of the optical disk is determined.
Aside from the process for determining the type of the optical disk, there is also a case that a recording layer reflectivity of the optical disk is measured, prior to reproducing the optical disk.
For example, the recording layer reflectivity of the optical disk is measured by irradiating the recording layer with a laser beam suitable for the type of the optical disk, and measuring PI (Pull-In: a sum signal of photo detector) level, and FE (Focus Error) level. Then, a gain of each servo amplifier is adjusted according to the reflectivity thus measured, thereby normalizing variation in reflectivity due to individual differences of the disk, and enhancing the precision in reading the signals.
Since there is a difference in reflectivity between a pit and a mirror surface (or a land and groove) of the recording layer, the optical disk is rotated while measuring the reflectivity, so that the measurement can be carried out uniformly.
With reference to FIG. 10A to FIG. 10D, the reflectivity measurement process will be explained in the case where it is performed subsequently after the optical disk discrimination process is performed.
FIG. 10A shows a movement of the objective lens with respect to the optical disk in the optical disk discrimination process. In other words, the objective lens 13 moves in such a manner as approaching the optical disk 100 which is in halt condition. When the focus captures the recording layer, the discrimination process for the optical disk 100 can be completed.
Since the focus captures the recording layer in this state, the reflectivity may be measured subsequently. However, if the optical disk 100 is rotated for measuring the reflectivity, upward surface wobbling may occur. Then, as shown in FIG. 10B, there is a possibility that the focus is deviated from the recording layer. In this situation, if the objective lens 13 is moved downwardly so as to detect the recording layer, the focus separates from the recording layer, and does not hit the recording layer.
Considering the problem above, in order that the focus captures the recording layer of the optical disk 100 being rotating, it is necessary to move the objective lens into a direction further approaching the optical disk 100, from the status in which the focus captures the recording layer in halt condition. Then, by displacing the objective lens 13 downwardly, from the position being focused above the recording layer, it is possible for the focus to capture the recording layer as shown in FIG. 10C.
On the other hand, if the objective lens 13 is placed too close to the optical disk 100, there is a possibility, as shown in FIG. 10D, that the surface of the optical disk 100 too much approaches the objective lens 13 at the time of downward surface wobbling caused by rotating the optical disk 100.
Therefore, appropriate control is required when the objective lens 13 is brought closer to the optical disk.
However, two-axle actuator which moves the objective lens 13 has variable sensitivities with respect to each individual object and variable outputs from the two-axle drive circuit, as well as there is a change in sensitivity due to the environment, such as temperature, installation status of the optical disk drive, or the like. Since the travel distance of the objective lens 13 may be affected by the difference in sensitivity of the two-axle actuator, there is a possibility that the objective lens 13 may not be controlled appropriately.